Continuous process for scrubbing so2 from a gas stream containing so2 and o2

ABSTRACT

AN INPUT STREAM CONTAINING S02 AND 02 IS CONTINUOUSLY TREATED IN ORDER TO REMOVE A SUBSTANTIALLY PORTION OF THE S02 THEREFROM BY THE STEPS OF: (A) SCRUBBING THE INPUT GAS STREAM WITH AN AQUEOUS ABSORBENT CONTAINING AN ALKALINE REAGENT AND A FIRST REDUCING AGENT TO PRODUCE A TREATED GAS STREAM AND A RICH ABSORBENT CONTAINING A MIXTURE OF SULFITE AND THIOSULFATE COMPOUNDS; (B) TREATING AT LEAST A PORTION OF THE RESULTING RICH ABSORBENT STREAM WITH A SECOND REDUCING AGENT AT CONDITIONS SELECTED TO COVERT SUBSTANTIALLY ALL OF THE REMAINING SULFITE COMPOUND CONTAINED THEREIN TO THE CORRESPONDING THIOSULFATE COMPOUND; (C) REACTING AT LEAST A PORTION OF THE RESULTING THIOSILFATERRICH WATER STREAM WITH CARBON MONOXIDE AT REDUCTION CONDITIONS SELECTED TO PRODUCE THE CORRESPONDING SULFIDE COMPOUNDS; (D) STRIPPING HYDROGEN SULFIDE FROM AT LEAST A PORTION OF THE RESULTING SULFIDE-RICH SOLUTION TO FORM A REGENERATED AQUEOUS ABSORBENT STREAM; AND, THEREAFTER, (E) PASSING AT LEAST A PORTION OF THE REGENATED ABSORBENT STREAM OF THE S02-SCRUBBING STEP. THE PRINCIPAL UTILITY OF THIS S02-SCRUBBING PROCESS IS ASSOCIATED WITH THE PROBLEM OF CONTINUOUSLY REMOVING A SULFUR DIOXIDE CONTAMINANT FROM FLUE OR STACK GAS STREAMS CONTAINING S02 AND 02 SUCH AS ARE TYPICALLY PRODUCED IN MODERN ELECTRICAL POWER GENERATING STATIONS IN ORDER TO ABATE A SERIOUS POLLUTION PROBLEM AND TO ENABLE THE SAFE, NOPOLLUTING BURNING OF WHICH SULFUR FUELS.

ept. 24, 1974 P. URBAN 3,338,191

CON'IlNLJOUS PROCESS FOR SCRUBBING S0 FROM A GAS STREAM CONTAINING 50 AND 0 Filed Marcn 12, 1973 Zone Second Read/0n E c t e D 52% Q 5 i t "3 I Q 2 y Scrubb/n United States Patent US. Cl. 423242 22 Claims ABSTRACT OF THE DISCLOSURE An input gas stream containing S0 and O is continuously treated in order to remove a substantial portion of the S0 therefrom by the steps of z (a) scrubbing the input gas stream with an aqueous absorbent containing an alkaline reagent and a first reducing agent to produce a treated gas stream and a rich absorbent containing a mixture of sulfite and thiosulfate compounds; (b) treating at least a portion of the resulting rich absorbent stream with a second reducing agent at conditions selected to convert substantially all of the remaining sulfite compound contained therein to the corresponding thiosulfate compound; (c) reacting at least a portion of the resulting thiosulfaterich water stream with carbon monoxide at reduction conditions selected to produce the corresponding sulfide compound; ((1) stripping hydrogen sulfide from at least a portion of the resulting sulfide-rich solution to form a regenerated aqueous absorbent stream; and, thereafter, (e) passing at least a portion of the regenerated absorbent stream to the SO -scrubbing step. The principal utility of this SO -scrubbing process is associated with the problem of continuously removing a sulfur dioxide contaminant from fiue or stack gas streams containing S0 and such as are typically produced in modern electrical power generating stations in order to abate a serious pollution problem and to enable the safe, nonpolluting burning of high sulfur fuels.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my prior, copending application Ser. No. 226,018 filed Feb. 14, 1972 which in turn is a continuation-in-part of my prior application Ser. No. 9,968, filed Feb. 9, 1970 and issued as US. Pat. No. 3,644,087 on Feb. 22, 1972.

DISCLOSURE The subject of the present invention is a novel, continuous process for the selective removal of S0 from a gas stream containing S0 and O More precisely, the present invention involves an SO scrubbing step, operated with an aqueous absorbent containing an alkaline reagent and a thiosulfate-producing reducing agent to form a rich absorbent stream containing a mixture of sulfite and thiosulfate compounds, coupled with a unique regeneration procedure which comprises: a preliminary treatment step wherein the remaining sulfite compound contained in the rich scrubbing solution is selectively converted to the corresponding thiosulfate compound, thereby forming a thiosulfate-rich aqueous stream; a primary reduction step wherein the thiosulfate contained in the resulting thiosulfate-rich stream is selectively converted to the corresponding sulfide compound, thereby forming a sulfide-rich stream; and an H 8 stripping step wherein the absorbent solution is regenerated with liberation of hydrogen sulfide by stripping the sulfide-rich stream. An

Patented Sept. 24, 1974 outstanding advantage of the disclosed procedure is that production of undesired, intractable sulfate by-products is controlled and minimized in both the scrubbing and regeneration sections of the process. In one important aspect, the present invention has to do with an SOg-scrubbing process to which is charged a gas stream containing S0 and O and which uses a highly efficient aqueous absorbent comprising an alkaline reagent such as sodium sulfite, hydroxide, carbonate or bicarbonate and a first reducing agent such as finely divided sulfur, sodium polysulfide, or sodium hydrosulfide, wherein the rich scrubbing solution withdrawn from the scrubbing step contains a mixture of sodium sulfite, sodium bisulfite and sodium thiosulfate, wherein a preliminary treatment step involves selective conversion of the residual sodium sulfite and sodium bisulfite with a second reducing agent such as finely divided sulfur, sodium polysulfide, or sodium hydrosulfide to sodium thiosulfate, wherein a primary reduction step involves selective conversion of the resulting sodium thiosulfate-rich stream to a sodium hydrosulfide-rich stream, wherein H 8 is stripped from the resulting solution with CO to form a regenerated absorbent stream and wherein the production of undesired, refractory sodium sulfate by-product is controlled and minimized in each step of the process.

A major problem encountered in many areas of industry today is associated with the production of waste gas streams containing sulfur dioxide and oxygen. The problem essentially involves the disposal of these waste gas streams without causing substantial air pollution. This problem is an extremely complex one primarily because of the wide variety of industrial sources that emit these sulfur dioxide-containing gas streams. One of the more common sources is associated with the combustion of sulfur-containing fuels in boilers, internal combustion engines, heating units, etc., to produce fiue or stack gas streams containing significant amounts of sulfur dioxide and oxygen. Similarly, waste gas streams of this type are generally produced by other industrial processes such as the smelting of sulfur-bearing ores, the refining of coke, the production of sulfur in a Claus process, the production of sulfuric acid, the sulfonation of organic material, the production of sulfur-containing fertilizer, the production of paper via a wood pulping process and the like industrial processes. It is well known that the indiscriminate discharge of these gas streams into the atmosphere results in a substantial air pollution problem because the sulfur dioxide has extremely detrimental effects on animal and plant life. In addition, the discharge of these gas streams into the atmosphere constitutes a waste of a valuable material because the sulfur contained in same is an industrial commodity. Many processes have been proposed for removal of sulfur dioxide from these gas streams. A large percentage of these proposed removal procedures involve contacting the sulfur dioxide containing gas stream with an aqueous absorbent stream which typically contain materials which chemically or physically react with the sulfur dioxide in order to absorb same into the liquid solution. A widely studied procedure involves the use of a solution of an alkaline reagent such as ammonium hydroxide, sulfate or carbonate, one of the alkali or alkaline earth metal salts such as sodium hydroxide, sulfite or carbonate, potassium hydroxide, sulfite or carbonate and the like alkaline reagents, to produce a rich absorbent stream containing the corresponding sulfite and/or bisulfite compound. For example, the use of an aqueous absorbent containing sodium bicarbonate and carbonate to form a rich scrubbing solution containing sodium sulfite and bisulfite.

Although the simple concept of the scrubbing S0 from the gas stream containing same with an aqueous absorbent containing an alkaline reagent has many advantages associated with it such as simplicity, high effectiveness and versatility, widespread adoption of this type of solution to the S pollution problem has been inhibited by the lack of a regeneration procedure for the rich absorbent stream which can continuously regenerate the rich absorbent stream by selectively and economically converting the absorbed S0 to a conveniently handled and saleable product in a highly selective manner. That is, it is required that the regeneration procedure enable the operation of the scrubbing process in a closed-loop system with respect to the absorbent. In particular, it is required that an acceptable regeneration procedure have the capability of not only continuously producing a regenerated absorbent stream but also minimizing undesired by-products so as to prevent the buildup of undesired intractable, difficulty removed ingredients in the absorbent stream once the system is operated in a closed-loop fashion. The by-product that is of the greatest concern in this regard is sulfatefor example, in a system using an aqueous solution of ammonium hydroxide or ammonium carbonate as the absorbent, ammonium sulfate and bisulfate salts once formed in the system can present serious problems if special means are not provided to remove it or its production is not suppressed or controlled. Specifically, these salts can build up until finely divided solids are formed which then can cause corrosion, erosion and fouling problems.

One solution that has been proposed to the problem of regenerating the rich absorbent streams of the types discussed above is the use of the suitable reducing agent to react with the sulfite compounds contained therein in order to selectively produce elemental sulfur and/or the corresponding sulfide compound. However, despite stringent precautions, when common reducing agents such as hydrogen, a suitable sulfide compound, or carbon monoxide are used in an attempt to directly reduce these sulfite compounds to elemental sulfur or the corresponding sulfide compounds, undesired sulfate compounds are formed in unacceptable amounts. These sulfate compounds are believed to be caused by the sulfite compounds undergoing auto-oxidation-reduction at the conditions necessary for direct reduction.

Even if sulfate by-product formation can be controlled as suppressed in the regeneration of the rich absorbent stream from the SO -scrubbing step, it does not necessarily mean that the problem of unchecked sulfate formation has been eliminated for all types of input SO -containing gas stream. Specifically, when the input gas stream to the SO -scrubbing step contains both S0 and 0 the formation of substantial amounts of sulfate by-products in the SO -Scrubbing step is inevitably observed when it is operated with conventional aqueous absorbents. The formation of this sulfate by-product in the scrubbing step is attributed to the interaction of the oxygen contained in the input gas stream with the water-soluble sulfite compound which is formed when the S0 is absorbed in the scrubbing solution. Hence in the special case where the input gas stream contains both S0 and 0 there is a substantial need for an $0 scrubbing process that operates with a closed aqueous absorbent loop and has the capability of controlling or suppressing sulfate formation not only in the regeneration section but also in the scrubbing section.

The problem addressed by the present invention is, therefore, to provide a flue gas scrubbing system for an input gas stream containing both S0 and 0 which is capable of continuous operation with a closed-loop absorbent circuit, which can selectively produce an easily removed substance as the principal product of the regeneration section and which can minimize the amount of undesired sulfate by-products formed in both the scrubbing section and the regeneration section.

I have now found a combination process for continuously scrubbing S0 from a gas stream containing both S0 and 0 which utilizes a conventional aqueous absorbent stream in a closed-loop fashion and which comprises a novel wet scrubbing step coupled with a novel regeneration procedure which enables the recovery of hydrogen sulfide in high yield, minimizes undesired sulfate by-products from both the scrubbing and the regeneration sections and produces a regenerated absorbent stream which is of a relatively low total sulfur content and, consequently, possesses a high capacity for S0 removal. The concept of my invention is based on my findings that (1) the sulfite compound formed in the scrubbing step by interaction between the S0 in the gas stream and the alkaline reagent can be selectively converted at conventional scrubbing conditions to the corresponding thiosulfate compound via reaction between a reducing agent, selected from the group consisting of finely divided elemental sulfur, a water-soluble sulfide compound, a polysulfide compound and mixtures thereof, at a much faster rate than the sulfite compound will react with the O component of the gas stream to form the undesired sulfate by-prodnot; (2) the thiosulfate compound once formed has a very small propensity to be oxidized by the O component of the gas to sulfate relative to the propensity of sulfite; (3) not all of this absorbed S0 need be converted to the thiosulfate in the scrubbing step in order to achieve a substantive diminution of the sulfate by-product of the scrubbing step; (4) operating with only a portion of the absorbed S0 converted to thiosulfate in the scrubbing step minimizes the danger that the reducing agent added thereto will contaminate the treated gas stream with sulfide; (5) conversion of the unreacted sulfite component of the rich scrubbing solution withdrawn from the scrubbing step can be finished in a preliminary treatment of the rich absorbent at relatively low severity conditions in a highly selective manner with additional reducing agent; and (6) the resulting thiosulfate-rich stream can be reduced by carbon monoxide in a highly selective, economic and efficient manner to form a solution of the corresponding sulfide compound from which hydrogen sulfide can be easily stripped to form the regenerated absorbent. Thus the central point of the present process involves using an 50 scrubbing procedure to make a rich absorbent stream containing a mixture of sulfite and thiosulfate coupled with a regeneration procedure wherein thiosulfate is used as an intermediate in a multi-step operation designed to convert the S0 contained in the input gas stream to hydrogen sulfide, rather than attempt to form only sulfite in this scrubbing step and thereafter directly reduce the resulting sulfite compound to sulfide in a single step operation. This sulfite-to-thiosulfate-to-sulfide route provides an S0 scrubbing system which facilitates careful control of sulfate by-product formation during both the scrubbing and regeneration operation, minimizes the risk of substantial contamination of the treated gas stream with sulfide, and enables the production of a regeneration absorbent stream which can be directly recycled to the SO -scrubbing step, thereby allowing the system to be operated in the closed loop fashion with respect to the absorbent stream.

It is accordingly, an object of the present invention to provide a simple, effective, efficient, and selective procedure for treating a gas stream containing S0 and 0 which process can selectively produce hydrogen sulfide and operate with a continuous closed-loop circuit of absorbent between the scrubbing section and the regeneration section. Another object is to minimize the amount of undesired, intractable by-products formed in both the regeneration and scrubbing sections of such a procedure. Another object is to provide a regeneration procedure for an SO scrubbing step that maximizes the sulfur difierential across the regeneration procedure, thereby increasing the capacity and efiiciency of the regenerated absorbent.

In brief summary, one embodiment of the present invention is a process for treating an input gas stream containing S0 and O in order to continuously remove a substantial portion of the S0 therefrom. The first step is a scrubbing step wherein the input gas stream is contacted in a suitable liquid-gas contacting zone With an aqueous absorbent containing an alkaline reagent and a first reducing agent selected from the group consisting of finely divided sulfur, a polysulfide compound, a watersoluble sulfide compound and mixtures thereof, at scrubbing and thiosulfate production conditions selected to result in a treated gas stream containing a substantially reduced amount of S0 and in an effluent water stream containing a mixture of a Water-soluble sulfite compound and a water-soluble thiosulfate compound. The next step is a preliminary treatment step which involves contacting at least a portion of the efiluent stream from the scrubbing step with a second reducing agent, selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof, at thiosulfate production conditions selected to form a substantially sulfite-free efiluent stream containing a thiosulfate compound. Thereafter, at least a portion of the effluent stream from the preliminary treatment step is reacted with carbon monoxide in the primary reduction step at reduction conditions selected to produce a sulfidecontaining aqueous effluent stream. The next step is a stripping step wherein hydrogen sulfide is stripped from at least a portion of the aqueous efiluent stream from the primary reduction step to produce a hydrogen sulfide-containing overhead stream and regenerated aqueous absorbent stream. In the final step, at least a portion of the resulting regenerated aqueous absorbent stream is passed to the scrubbing step, thereby providing a closed-loop flow circuit of absorbent.

In another embodiment, the invention is a process as outlined above in the first embodiment wherein the alkaline reagent utilized in the aqueous absorbent stream is selected from the group consisting of the salts of ammonia, the alkali metals and the alkaline earth metals which hydrolyze in water to form a basic soluti0nfor example, the hydroxide sulfite and carbonate salts. The term basic here is used to connote a solution capable of absorbing S0 by formation of sulfite or bisulfite salts. In a more specific embodiment, the present invention is a process for treating a gas stream containing S0 and O in order to remove a substantial portion of the S0 therefrom. The first step in this embodiment involves a scrubbing step in which the input gas stream is contacted with an aqueous absorbent containing sodium carbonate, bicarbonate or sulfite and a first reducing agent selected from the group consisting of elemental sulfur, sodium polysulfide, sodium hydrosulfide and mixtures thereof at scrubbing and thiosulfate production conditions selected to form a treated gas stream containing a substantially reduced amount of S0 and an effluent water stream containing a mixture of sodium sulfite or bisulfite and sodium thiosulfate. At least a portion of the effluent water stream from the scrubbing step is thereafter contacted in a preliminary treatment step, with a second reducing agent, selected from the group consisting of finely divided sulfur, sodium polysulfide, sodium hydrosulfide and mixtures thereof, at thiosulfate production conditions selected to result in substantially sulfite-free effluent stream contain ing sodium thiosulfate. The primary reduction step then involves reacting at least a portion of the efiiuent stream from the preliminary treatment step with carbon monoxide at reduction conditions selected to produce an aqueous effluent stream containing sodium hydrosulfide. At least a portion of the aqueous efiiuent stream from this primary reduction step is thereafter subjected to contact with a carbon dioxide-containing stripping gas at stripping conditions etfective to produce a regenerated aqueous absorbent stream containing sodium bicarbonate or carbonate and an overhead gaseous stream containing hydrogen sulfide. At least a portion of the resulting regenerated absorbent stream is then in the final step, passed to the scrubbing step, thereby providing a closed-loop flow circuit of absorbent.

In another embodiment, the present invention is a process as described above in the last embodiment wherein the reducing agent utilized in the preliminary treatment step is hydrogen sulfide and wherein at least a portion of the overhead stream produced in the stripping step is passed to the preliminary treatment step in order to supply a portion of the hydrogen sulfide reactant utilized therein.

Yet another embodiment of the present invention involves a process as outlined above in the first embodiment wherein the primary reduction step is performed in the presence of a catalyst comprising a catalytically eifective amount of a metallic component selected from the group consisting of the transition elements of Groups VI and VIII of the Periodic Table, and compounds thereof, combined with a porous carrier material,

Other objects and embodiments of the present invention are hereinafter disclosed in the following detailed discussion of the input streams, the preferred conditions, the preferred reactants, the output streams and the mechanics associated with each of the essential and preferred steps of the present invention.

The starting point for the subject process is a scrubbing step wherein an input gas stream containing S0 and O is contacted in a suitable gas-liquid contacting means with an aqueous absorbent containing an alkaline reagent and a first reducing agent selected from the group consisting of elemental sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof. As previously explained, the input gas stream passed to this step is typically a flue or stack gas. For example, a typical stack gas stream containing about 1 to about 10% 0 about 5 to 15% or more CO and about 3 to about 10% or more H O, about 0.01 to about 1% or more S0 In many cases, the input gas stream will also contain carbon monoxide, oxide of nitrogen, entrained fly ash and the other well-known ingredients for flue gas streams. The amount of S0 contained in this input gas stream can vary over a Wide range; namely, from about 0.01 toabout 1 mole percent or more, with a more typical amount being about 0.05 to about 0.5 mole pcrcent. Likewise the amount of O in this stream can vary over a wide range of about 1 to about 20% and typically about 5 to 15%. In many cases, this input gas stream is available at a relatively high temperature of about 200 to about 500 F. or more, and since it is preferred that the temperature of the input gas stream be at a relatively low level because this increases the capacity of the absorbent solution, it is often advantageous to cool the input gas stream by any suitable means such as by pre saturating it with water under adiabatic conditions.

The aqueous absorbent utilized in this scrubbing step is generally characterized as an aqueous solution of a suitable alkaline reagent which reacts with water to give a solution which will absorb 50 such as ammonium hydroxide, ammonium carbonate and bicarbonate, ammonium sulfite, the alkali metal hydroxides, sulfites, carbonates and bicarbonates and the Water-soluble alkaline earth metal hydroxides, sulfites, carbonates and bicarbonates, and the like alkaline reagents. Of the alkali metal reagents, sodium hydroxide, sodium carbonate, sodium sulfite, sodium bicarbonate, potassium hydroxide, potassium carbonate, potassium bicarbonate are particularly preferred. In most cases, excellent results are obtained when the alkaline reagent is ammonium or sodium hydroxide, sulfite, carbonate or bicarbonate. It is to be noted that it is within the purview of the present invention to use a mixture of the alkaline reagents previously mentioned. Since it is also contemplated that the scrubbing step can be operated with the absorbent continuously cycling around the scrubbing means, it is possible that absorbent could accumulate substantial amounts of sulfite and bisulfite compounds. In this last case, only a small portion of the rich elfiuent stream from the scrubbing step would be sent to the regeneration section of the process, and the major portion of the rich absorbent would be commingled with the regenerated absorbent stream and recycled to the scrubbing step.

According to the present invention, the absorbent stream also contains a first reducing agent selected from the group consisting of elemental sulfur, a water-soluble sulfide compound, a polysulfide compound and mixtures of these. The characteristic of each of these reducing agents and their method of use are hereinafter discussed in detail in the section of this specification dealing with the second reducing agent. These details are equally applicable to use of the first reducing agent and will not be repeated here. A key feature of my process is that the amount of first reducing agent used is about 10% to about 90% of the stoichiometric amount, and preferably about 30 to 60%, that would be necessary to convert all of the S absorbed in the scrubbing step to thiosulfate. That is, the amount of first reducing agent utilized is selected so that the efiluent from the scrubbing step always contains a mixture of unreacted sulfite and thiosulfate. The chief advantage associated with this mode of operation is that it minimizes the risk of contaminating the treated gas stream with the first reducing agent. It is to be noted that the first reducing agent need not be identical to the second reducing agent-for example, the first reducing agent can be sodium hydrosulfide and the second sodium polysulfide or vice versa. The preferred mode of operation is as shown in the attached drawing-namely, the first reducing agent is a water-soluble hydrosulfide and the second is hydrogen sulfide.

The source for the first reducing agent may be external or internal to my process. The preferred source is an internal one and generally can be either a portion of the effiuent stream from the primary reduction step or a portion of the H S-rich overhead stream from the sulfide stripping step. Another internal source for the first reducing agent is a portion of the sulfide charged to the sulfide stripping step which can be carried back to the scrubbing step by means of the regenerated absorbent stream if the stripping conditions are selected so that the required amount of sulfide for operating the scrubbing step is left in the bottom stream from the H 5- stripping zone.

The first reducing agent can be introduced into the SO -scrubbing step in any suitable manner, either in admixture with the input gas stream or the absorbent stream or independently of both of these. The preferred procedure is to admix it with the absorbent stream prior to its introduction into the scrubbing step. Another procedure is to introduce the first reducing agent at a point substantially below the point of introduction of the ab sorbent stream in order to prevent carryover of the first reducing agent from the top of the scrubbing zone. In the mode of operation wherein the first reducing agent is a portion of the overhead stream from the H 5- stripping step, the preferred practice is to admix it with the input gas stream prior to introduction of same into the SO -scrubbing step.

The amount of alkaline reagent contained in the scrubbing solution is subject to some choice depending upon the specific requirements associated with the particular gas stream being treated; ordinarly, acceptable results are obtained when the alkaline reagent comprises about 1 to about 50 wt. percent of the absorbent solution, and more preferably to 1 to about 15 wt. percent. Of course, absorbent solutions containing an amount of the alkaline reagent up to the solubility limit of the particular alkaline reagent selected at the conditions maintained in the scrubbing step can be utilized if desired. In the case Where the absorbent is continuously cycled around the scrubbing step and only a drag stream drawn off for regeneration, the total amount of the alkaline reagent contained in the solution (i.e. fresh and spent) can reach rather high levels; for example, it can easily constitute 5 to about 50 Wt. percent of the absorbent solutions. When the alkaline reagent is sodium sulfite, excellent results are obtained when the scrubbing solution pontains about 5 to wt. percent thereof.

This scrubbing step can be carried out in a conventional scrubbing zone in any suitable manner including multiple stages. The scrubbing solution can be passed into the scrubbing zone in either upward or downward flow and the input gas stream can be simultaneously introduced into the scrubbing zone in concurrent or countercurrent flow relative to the scrubbing solution. A particularly preferred procedure involves downward fiow of the scrubbing solution with counter-current flow of the gas stream which is to be treated. The scrubbing zone is preferably a conventional gas-liquid contacting zone containing suitable means for effecting intimate contact between a descending liquid stream and an ascending gas stream. Suitable contacting means include bubble trays, bafiles, and any of the various packing materials known to those skilled in the art. In this countercurrent mode of operation, a treated gas stream is withdrawn from the upper region of the scrubbing zone and a rich absorbent solution is withdrawn from the lower region thereof. For the class of alkaline reagents of concern here, the rich absorbent solution will contain substantial amounts of a Water-soluble sulfite compound such as ammonium sulfite and/or bisulfite, sodium sulfite and/or bisulfite and the like. As indicated previously, according to one mode of operation of the scrubbing step, only a drag stream from the rich absorbent withdrawn from the step is sent to the regeneration section of the process; the rest is cycled around the scrubbing step. The drag stream is ordinarily withdrawn at a rate at least sufficient to continuously remove the net sulfur taken up in the scrubbing step.

In accordance with the present invention, this scrubbing step is conducted under S0 scrubbing and thiosulfate production conditions which are selected on the basis of the characteristics of the specific alkaline reagent utilized, the sulfur dioxide content of the input gas stream, the portion of the sulfur dioxide that is to be removed in the scrubbing step, and the physical properties of the scrubbing zone. Ordinarily, the scrubbing step is preferably operated at a relatively low temperature of about 10 to 100 C., and preferably about 25 to about C. a relatively low pressure which typically approximates atmospheric; a volume ratio of input gas streams to scrubbing solution of about 25:1 to about 10,000:1 and preferably about :1 to about 300:1, and a pH of about 4 to 7 or more and preferably about 5.5 to about 7. When the input gas stream is a fiue or stack gas steam, means, must ordinarily be provided for cooling the input gas stream to a relatively low temperature of about 10 to 100 C. before it is introduced into the scrubbing step. Likewise, since the typical operation of the scrubbing step involves the handling of large volumes of gas containing only a relatively small amount of sulfur dioxide, it is preferred that the pressure drop through the scrubbing zone be held to a minimum so as to avoid the necessity of compressing large volumes of gas to overcome the pressure drop within the scrubbing zone, therefore, it is preferred to run the scrubbing zone gas-full instead of liquid-full.

Following the scrubbing step, the next step of the present process is the preliminary treatment step and it involves the conversion, in the highly selective manner, of the remaining sulfite or bisulfite compound contained in at least a portion of the efiiuent water stream withdrawn from the scrubbing step, to the corresponding thiosu-lfate compound. Ordinarily the sulfite or bisulfite compound is contained in the feed stream to this step in an amount of about 0.01 wt. percent, calculated on an equivalent sulfur basis, up to the solubility limit of the particular sulfite compound in water at the conditions utilized in the scrubbing step; for example, the feed stream to this step can contain about 1 to about 20 wt. percent sulfur as ammonium or sodium sulfite and/or bisulfite and more typically about 5 to 10 wt. percent. According to the present invention, this step involves a reaction between the sulfite compound contained in this rich absorbent stream and a second reducing agent. This second reducing agent is selected from the group consisting of finely divided sulfur, a polysulfide compound, a Water-soluble sulfide compound and mixtures thereof. In the first mode of operation of this step, finely divided sulfur is utilized as the second reducing agent, and it is preferred that the sulfur be present in particle size of about 10 to about 250 microns, with best results obtained with particles of about 25 to about 100 microns. Typically, it is a good practice to introduce the sulfur into this step via a water stream containing a slurry of finely divided sulfur in an amount of about 1 to about 75 wt. percent thereof, although many other suitable means for injecting finely divided solid particles can be utilized if desired. In this first mode of operation, it is preferred to also utilize a wetting agent in the reaction mixture in order to facilitate good contact of the elemental sulfur with the sulfite compound. Suitable wetting agents are: the salts of alkyl aryl sulfonates such as the sodium salt of dodecylbenzene sulfonate; sulfonated fatty acid esters; C to C alkyl sulfates: C to C alkyl sulfonates; alkyl polyoxyethylene alcohols; ethylene oxide condensation products of alkyl phenols; quaternary ammonium salts such as octadecyldimethylbenzyl ammonium chloride and the like wetting agents. The wetting agent is preferably utilized in a relatively small amount corresponding to about 0.01 to about 1 wt. percent of the sulfite compound that is reacted. The amount of elemental sulfur utilized in this first mode of operation of this preliminary treating step should be sufficient to supply one atom of sulfur per molecule of sulfite compound contained in the portion of the effluent water stream from the SO -scrubbing step, with the preferred amount corresponding to about 1 to about 3 atoms of sulfur per mole of sufite compound.

In a second mode of operation for this preliminary treatment step, the second reducing agent is a polysulfide compound. Suitable polysulfide compounds include the ammonium, alkali metal, and alkaline earth polysulfides. Best results are ordinarily obtained with ammonium or sodium polysulfide. The polysulfide compound is ordinarily charged to this step in the form of an aqueous solution containing about 1 to about 50 wt. percent of the polysulfide compound. It is to be noted that when the reducing agent is a polysulfide compound, no wetting agent is necessary in order to achieve good contact with the sulfide compound. The amount of the polysulfide compound charged to this step is preferably sufiicient to provide at least the stoichiometric amount necessary for the reaction between it and the sulfite compound to produce the corresponding thiosulfate compound. In the typical case where the polysulfite compound contains four atoms of elemental sulfur and one atom of sulfide (e.g. (NH S the stoichiometric amount is /6 moles of polysulfide per mole of sulfite compound, with a preferred value being about M1 to about A or more moles of polysulfide per mole of sulfite compound.

In a third mode of operation of this preliminary treatment step, the second reducing agent is a water-soluble sulfide compound. Suitable water-soluble sulfide compounds are hydrogen sulfide, ammonium sulfide, ammonium hydrosulfide and the sulfides and hydrosulfide salts of the alkali and alkaline earth metals. Best results are ordinarily obtained in this mode of operation of this step when the sulfide reactant is hydrogen sulfide or ammonium hydrosulfide. The amount of this sulfide reactant utilized in this step is at least sufiicient to provide 0.5 moles of sulfide compound per mole of sulfite compound contained in the portion of the efiluent stream from the SO -scrub'bing step, with best results obtained at a mole ratio corresponding to about 0.6 to about 1.5 or more. Likewise, in this third mode of operation, good results are ordinarily obtained when the pH of the input water stream is in the range of 4 to about 7. Internal sources for the necessary sulfide reactant in this mode of operation are: (1) a portion of the effluent stream from the primary reduction step, (2) a portion of the overhead stream from the sulfide stripping step, and (3) a portion of the bottom stream from the sulfide stripping step when it is operated to only strip a portion of the sulfide charged thereto.

Conditions utilized in this preliminary treatment step can be generally described as thiosulfate production conditions and comprise: a temperature of about 20 to about C., a pressure sufiicient to maintain the input water stream in the liquid phase and a contact time corresponding to about 0.05 to 1 or more hours. In general, the contact time necessary for the desired reaction is a function of the reducing agent utilized, with relatively short contact times of about 1 to 5 minutes being sutficient in the case where the reducing agent is a sulfide or a polysulfide compound. The other two reducing agents require a relatively longer contact time ranging up to about 0.1 to about 1 hour. Considering all of the factors involved in the operation of this preliminary treatment step, best results are ordinarily obtained when the reducing agent is hydrogen sulfide or a polysulfide compound, particularly sodium or ammonium polysulfide.

Following this preliminary treatment step, a substantially sulfite-free aqueous efiluent stream containing relatively large amounts of a thiosulfate compound is withdrawn therefrom and at least a portion of it is passed to the primary reduction step of the present process wherein thiosulfate is reacted with carbon monoxide at reduction conditions selected to produce the corresponding sulfide compound.

The carbon monoxide stream for use herein may be obtained from any suitable source or may be prepared in any suitable manner. An acceptable carbon monoxide stream is obtained by the partial oxidation of organic materials, and particularly carbon at high temperature with oxygen, air or steam. Likewise, a carbon monoxide stream suitable for use herein can be prepared by the reduction of carbon dioxide by hydrogen, carbon or certain metals at high temperatures. For example, a gas stream containing about 40% carbon monoxide is easily prepared by blowing steam through a bed of coal at an elevated temperature. Another suitable carbon monoxidecontaining stream is obtained by simultaneously blowing air and steam through a bed of red hot coal to produce a gas stream containing about 30% carbon monoxide. In addition, blast furnace gases resulting from the reduction of iron oxide by red hot coke can be utilized to supply the necessary carbon monoxide stream if desired. Yet another source of a suitable carbon monoxide stream is a stream prepared by passing carbon dioxide and oxygen through charcoal or coke at a temperature greater than about 1,000 C. in order to decompose the CO to CO. Regardless of the source of the carbon monoxide, it is preferably used herein in an amount sufficient to provide a mole ratio of carbon monoxide to thiosulfate compound of at least 4:1, with best results obtained at a mole ratio of about 5:1 to 10:1 or more. I have observed that the amount of sulfide formed increases with higher mole ratios of carbon monoxide to thiosulfate.

This primary reduction step can be carried out, if desired, without the use of a catalyst; however, in many cases, it is advantageous to use a catalyst for this reaction. Based on my investigations I have determined that improved results are obtained in this step when the reaction zone contains materials such as particles of charcoal, and particles of activated carbon. Particularly good results are obtained with a catalyst comprising a catalytically efiective amount of a metallic component selected from the group consisting of the transition metals of Group VI and VIII such as chromium, molybdenum, tungsten, iron, cobalt, nickel, platinum, palladium, etc. From this, I have concluded that preferred catalysts for the desired reduction reaction comprise a combination of a Group VI or a Group VIII transition metal component with a suitable porous support such as alumina or activated carbon particularly preferred embodiments of the present step involve the use of catalysts in which the metallic component is present in the form of a metallic sulfide such as cobalt sulfide, or molybdenum sulfide, or tungsten sulfied combined with a carrier material. The preferred carrier materials are activated carbons such as those commercially available under the trade names of Norite, Nuchar, Darco and other similar products. In addition, other conventional natural or synthetic highly porous inorganic carrier materials may be used as the support for the metallic component such as alumina, silica, silica-alumina, etc. Best results are ordinarily obtained with a catalyst comprising cobalt or molybdenum or tungsten sulfide combined with relatively small particles of activated carbon. Excellent results have been obtained with to 12 mesh activated carbon particles containing about 5 wt. percent of cobalt sulfide. In general, the amount of the metallic component utilized in the catalyst can be any amount which is catalytically effective and ordinarily should be sufficient to comprise about 0.1 to about 50% thereof, calculated on a metallic sulfide basis. These catalysts can be prepared according to any of the conventional procedures for combining a metallic component with a carrier material, with an impregnation procedure with a soluble, decomposable compound of the desired Group VI or VIII transition metal ordinarily giving best results.

This primary reduction step can be carried out in a conventional reaction zone in any suitable manner. The thiosulfate-containing eflluent stream from the preliminary treatment step can be passed into the reaction zone in either upward, radial or downward fiow and the carbon monoxide stream can be simultaneously introduced into the reaction zone in either countercurrent or concurrent flow relative to the thiosulfate-containing efiluent stream. In particular, a preferred embodiment of this step involves downward flow of the thiosulfate stream with countercurrent flow of the carbon monoxide stream. It is preferred to utilize suitable means in the reaction zone for effecting intimate contact between a liquid stream and a gas stream. Suitable contacting means include bubble trays, battles and any of the various packing materials known to those skilled in the art. In the preferred case where a catalyst is utilized in this second step, best results are ordinarily obtained when the catalyst is maintained within the reaction zone as a fixed bed of relatively small particles. These catalyst particles perform the dual functions of catalyzing the desired reaction and of promoting intimate contact between the gas and liquid streams. In the preferred countercurrent flow mode of operation for this step, a gas stream containing carbon dioxide, unreacted carbon monoxide and some hydrogen sulfide is typically withdrawn from the upper region of the reaction zone. Likewise, an aqueous eflluent stream containing the corresponding sulfide compound is withdrawn from the lower region of the reaction zone. For example, in the case where the input stream to this primary reduction step contains sodium thiosulfate, this aqueous efiiuent stream will primarly contain sodium hydrosulfide with minor amounts of unreacted sodium thiosulfate, sodium bicarbonate and sodium carbonate.

The reduction conditions utilized in this primary reduction step are typically relatively more severe than those utilized in the preliminary treatment step and can be generally characterized as reduction conditions sufiicient to effect conversion of thiosulfate to sulfide. The temperature is preferably selected from the range of about 100 to about 350 C., with best results obtained at a relatively high temperature of about 150 to about 250 C. It is an essential feature of the present invention that the step is conducted under liquid phase conditions; accordingly, the pressure employed must be sufficient to maintain at least a portion of the efiluent stream from the preliminary treatment step in the liquid phase. Typically the pressure is selected from the range of about 100 to about 3000 p.s.i.g. and preferably about 500 to 1000 p.s.i.g., as a function of the reaction temperature in order to maintain the desired liquid phase condition. Particularly good results are obtained at a temperature of about 200 C., and a pressure of about 750 p.s.i.g. It is preferred to use a liquid hourly space velocity (defined on the basis of the liquid volume charge rate of the effluent stream from the first step divided by the volume of the reaction zone utilized in this second step in the case where a catalyst is not utilized and by the volume of the catalyst bed in the case where a catalyst is used in this second step) selected from the range of about 0.25 to about 10 hr.- with best results obtained at about 0.5 to about 3 hrf Excellent results have been obtained in this step with a LHSV of 1 hlf In the next step of the present invention the sulfidecontaining aqueous efiluent stream recovered from the primary reduction step is subjected to a H S-stripping step designed to liberate hydrogen sulfide therefrom. Although any suitable stripping gas can be utilized including steam, nitrogen air and the like, carbon dioxide is particularly preferred, because it acts to decrease the pH of the solution and form the corresponding carbonate and bicarbonate salts. For instance in the case where the efiiuent stream from the primary reduction step contains sodium hydrosulfide, stripping with carbon dioxide liberates hydrogen sulfide and produces a bottom stream containing sodium carbonate and bicarbonate. In another mode of operation of this step, the efiluent water stream from the primary reduction step can be subjected to conditions sufiicicut to thermally decompose the sulfide compound contained in this stream. For example, in a typical case where this water stream contains ammonium hydrosulfide, acceptable decomposition conditions are a temperature of about to 200 C., and a pressure of about 1 to about 75 p.s.i.g. Typically this decomposition mode of operation is contacted in a conventional distillation zone wherein upflowing vapors are generated by supplying heat to the bottom of same by means such as a steam coil or conventional reboiler. Regardless of which mode of operation is employed in this stripping step, an overhead stream containing hydrogen sulfide will be produced. Likewise, a regenerated aqueous absorbent stream which is substantially reduced in total sulfur content will be recovered therefrom as a bottom stream.

This regenerated aqueous absorbent stream is substantially reduced in total sulfur content relative to the rich absorbent stream withdrawn from the scrubbing step and usually Will contain less than 10% of the amount of sulfur contained in the rich absorbent stream. In the case where the carbon dioxide is utilized in the stripping step as the stripping medium, this treated water stream will contain substantial amounts of the carbonate or bicarbonate salt of the alkaline reagent originally present in the input absorbent streamfor example in the case where the alkaline reagent is ammonia, the regenerated absorbent stream will contain ammonium carbonate and/ or bicarbonate, and in the case where the alkaline reagent is sodium hydroxide or carbonate the treated water stream will contain sodium carbonate and/ or bicarbonate.

In accordance with the instant process, at least a portion of the regenerated absorbent stream is passed to the SO -Scrubbing step. In one embodiment of the instant process, a portion of the hydrogen sulfide-containing overhead stream produced in the H s-stripping step is passed to the preliminary treatment step in order to supply at least a portion of the second reducing agent used therein. The remaining portion of this hydrogen sulfide-containing stream is then recovered as one of the product streams from the instant process. The hydrogen sulfide contained in this product stream can be converted to elemental sulfur by any suitable oxidation procedure such as a conventional Claus process or to sulfuric acid or used per se.

Having broadly characterized the essential steps comprising the present process, reference is now made to the attached drawing for a detailed explanation of working example of a preferred flow scheme for the present invention. The attached drawing is merely intended as a general representation of the flow scheme involved with no intention to give details about heaters, pumps, valves and the like equipment except where a knowledge of these devices is essential to an understanding of the present invention or would not be self-evident to those skilled in the relevant art.

Referring now to the attached drawing, a flue gas stream enters the system via line 1 and is passed into the lower region of a conventional liquid-gas scrubbing means, zone 2. In this zone it is passed in countercurrent flow to a descending stream of absorbent solution which enters the upper region of zone 2 via line 4. The input gas stream contains about 5% O 12% C0 6% H O, 76.8% N and 0.2% S0 Zone 2 is a conventional-gas liquid contacting zone fitted with conventional means such as baffles, trays, packing material and the like, for effecting intimate contact between an ascending gas stream and a descending liquid stream. Zone 2 is run gas-full in order to minimize pressure drop through this zone.

Also introduced into zone 2 is a liquid stream comprising the aqueous absorbent solution. It enters zone 2 via line 4 and is made up of two separate streams, one of which is a regenerated absorbent stream obtained from the regeneration section of the present system and the second of which is a major portion of the rich absorbent stream withdrawn from the lower region of zone 2 via line 4. The alkaline reagent present in this regenerated absorbent stream is primarily sodium carbonate with a minor amount of sodium bicarbonate. Because of the facts that the rich absorbent stream withdrawn from the bottom of zone 2 contains sodium bisulfite and that the rich absorbent stream is admixed with the regenerated absorbents stream prior to introduction into the scrubbing zone, the actual alkaline reagent used in zone 2 is primarily sodium sulfite. It is produced by a reaction between sodium bisulfite and sodium carbonate and bicarbonate that occurs in line 4 after the junction with line 15.

According to the mode of operation of zone 2 shown in the drawing, the regenerated absorbent stream comprises an alkaline reagent obtained from the bottom stream from H S-Stripping zone 13 and a first reducing agent which is a portion of the sulfide-containing effluent stream from zone 9. The former stream is withdrawn from zone 13 via line 15, commingled with a portion of the effluent stream from zone 9 at the junction of line 21 with line 15, and the resulting mixture is passed via lines 15 and 4 to the scrubbing zone 2. The stream added to the regenerated absorbent via line 21 contains the first reducing agent in the form of sodium hydrosulfide. The first reducing agent is added to regenerated absorbent in an amount sufficient to convert about 40% of the S0 entering zone 2 to sodium thiosulfate. This corresponds to an amount sufficient to react about 0.2 moles of sodium hydrosulfide per mole of S0 entering zone 2. Line 19 is used during startup to introduce an aqueous solution containing sodium carbonate or bicarbonate or hydroxide and sodium hydrosulfide until zones 9 and 13 are operating properly. Thereafter, makeup alkaline reagent is introduced via line 19 as needed.

The rich absorbent stream is withdrawn from the bottom of zone 2 via line 4 and a major portion of this stream is continuously cycled around zone 2 in order to allow the concentration of sulfite and thiosulfate salts in the absorbent stream to build to relatively high levels. This procedure increases the capacity of the absorbent and minimizes the amount of the absorbent stream that must be cycled through the regeneration section of the system. The absorbent stream introduced into scrubber 2 via line 4 will accordingly contain a substantial amount of sodium bisulfite and thiosulfate salts along with the alkaline reagent. Ordinarily zone 2 is operated by monitoring the pH of the absorbent stream at the inlet to zone 2 and controlling the amount of the rich absorbent stream diverted to the regeneraion section of the system at the junction of 14 lines 4 and 5 in response to a decrease in pH level. The preferred pH range is about 4 to about 7 or more, with best results ordinarily obtained in the range of about 5 to 7. With the system operating so that the absorbent stream introduced into zone 2 via line 4 is maintained at a pH level within this range, the rich absorbent stream withdrawn continuously from zone 2 via line 4 will typically contain about 1 to about 14 or more wt. percent sulfur principally as a mixture of sodium sulfite, sodium bisulfite and sodium thiosulfate. In addition, minor amounts of sodium sulfate are formed in zone 2. In the particular case shown in the drawing, the rich absorbent preferably contains about 2.9 wt. percent sodium bisulfite, about 8.3 wt. percent sodium sulfite, about 10.3 wt. percent sodium thiosulfate, and only about 5 wt. percent sodium sulfate.

Zone 2 is operated at a temperature of about 50 C., a pressure of about atmospheric and a gas to absorbent volume ratio of about 200:1. At these conditions, the treated gas stream withdrawn from the upper region of zone 2 via line 3 is found to contain less than 5% of the S0 originally present in the input gas stream.

As previously explained, the rich absorbent stream withdrawn from the lower region of zone 2 via line 4 is divided into two portions at the junction of line 5 with line 4. The major portion continues on via line 4 and is ad mixed with regenerated absorbent in an amount to provide about 0.7 mole of sodium carbonate per mole of S0 entering zone 2 at the junction of line 15 with line 4. The result ing mixture of cycled and regenerated absorbent is then reintroduced into zone 2 via line 4. The minor portion of the rich absorbent is passed via line 5 into the first reacting zone, zone 6. The amount of rich absorbent passed into zone 6 is ordinarily at least sufiicient to remove the net sulfur absorbed in zone 2 from the input gas stream in order to line out the concentration of sulfur in the absorbent stream. In the case under consideration, the amount of the absorbent withdrawn for regeneration via line 5 will be about 0.1 to 10%, and typically about 1%, of the rich absorbent stream withdrawn from the bottom of zone 2. It is to be noted that during start up of scrubbing zone 2, the inventory of the scrubbing solution and first reducing agent needed for initiating operation is introduced into the system via lines 19, 15 and 4. It is also to be recognized that there is a net water make in the regeneration of the absorbent, a major porion of which ordinarily is removed from the system in the treated gas stream withdrawn from the system via line 3.

The rich absorbent stream introduced into zone 6 via line 5 in the particular case of interest here contains about 2.9 wt. percent sodium bisulfite, about 8.3 wt. percent sodium sulfite, about 10.3 wt. percent sodium thiosulfate and about 5% sodium sulfate. It enters the upper region of zone 6 which, once again, is a conventional liquid-gas contacting zone designed to effect intimate contact with an ascending gas stream and a descending liquid stream. Also, introduced into zone 6 via line 14 is a gas stream containing the second reducing agent, hydrogen sulfide. During startup of zone 6 sufficient H 8 is introduced thereto via lines 18 and 14 to initiate the desired conversion reaction. Thereafter a portion of the hydrogen sulfide-containing gas stream which is produced in the subsequently described H S-Stripping step is passed to zone 6 from zone 13 via line 14 in order to furnish the required quantity of second reducing agent. In either case the amount of hydrogen sulfide supplied to zone 6 is sufiicient to react about 0.5 moles of H 5 per mole of sodium sulfite plus sodium bisulfite charged to zone 6. By conventional means zone 6 is maintained at a temperature of 50 C. and at a pressure of about 50 p.s.i.g. Also the flow rates of the stream into zone 6 are adjusted to provide a residence time of the liquid stream in zone 6 of about 0.5 hours.

An overhead gaseous stream, containing any unreacted hydrogen sulfide, carbon dioxide and trace amounts of carbon monoxide, is then withdrawn from zone 6 via line 7 and vented from the system. Likewise, an aqueous efilucut stream is withdrawn from the lower region of zone 6 via line 8 and at least a portion of it charged to the second reaction zone, zone 9. This aqueous effluent stream contains sodium thiosulfate in an amount corresponding to a conversion in zone 6 of greater than 95% of the input sodium sulfite and bisulfite to sodium thiosulfate. Furthermore, the amount of undesired sodium sulfate formed by the reaction in zone 6 is less than 3% of the input sulfite to this step. Accordingly, the aqueous efiiuent stream withdrawn from zone 6 via line 8 principally contains sodium thiosulfate with very minor amounts of unreacted sodium sulfite and bisulfite and of undesired sodium sulfate.

Although not shown in the attached drawing, it is within the scope of the present invention to subject the efiluent stream from zone 6 to any suitable procedure for separating sodium sulfate from sodium thiosulfate such as selective precipitation with lime or fractional crystallization. The resulting sulfate depleted stream is then being charged to zone 9. It is a feature of the present invention that the production of sodium sulfate is suppressed; however, it is not completely eliminated and an exit port for it must be provided if the process is to operate for a significant period.

Zone 9, the second reaction zone, is another liquid-gas reaction zone designed to effect intimate contact between an ascending gas stream and a descending liquid stream. At least a portion of the aqueous eflluent stream zone 6 is introduced into the upper region of zone 9 by means of line =8. Likewise, a carbon monoxide-rich stream is introduced into the lower region of zone 9 by means of line 11. Zone 9 contains a fixed bed of a catalyst com prising 10 to 12 mesh particles of activate carbon having a cobalt sulfide component combined therewith in an amount sufficient to result in a catalyst containing about wt. percent cobalt. The amount of carbon monoxide introduced into the system via line 11 corresponds to a carbon monoxide to sodium thiosulfate mole ratio of about 5.5: 1. The reduction conditions maintained in zone 9 by conventional means are a temperature of about 200 C., a pressure of about 750 p.s.i.g. and a liquid hourly space velocity of 1 hr.-

A sulfide-containing aqueous efiiuent stream is then withdrawn from the lower region of zone 9 via line 12. It is divided into two portions at the junction of line 12 with line 21. The first portion continues on via line 12 and is passed to stripping zone 13. The second portion is passed to zone 2 by means of lines 21, 15 and 4 as previously explained. An overhead gaseous stream is similarly withdrawn from the upper region of zone 9 via line and passed to the lower region of stripping zone :13. An analysis of the aqueous stream withdrawn from zone 9 via line 12 indicates that 99% of the sodium thiosulfate charged to zone 9 is converted therein to sodium hydrosulfide. A similar analysis of the overhead gas stream indicates that it contains a relatively large amount of carbon dioxide with minor amounts of unrcacted carbon monoxide, hydrogen sulfide, water and inerts. At the junction of line 10 with line 16 additional quantities of CO may be added to the system in order to increase the efficiency of the stripping operation in zone 13. In many cases the amount of CO contained in the overhead stream from zone 9 is suflicient for the stripping step and the addition of CO via line 16 is not necessary.

In stripping zone 13, the aqueous effluent stream from zone 9 is countercurrently contacted with an ascending gaseous stream which essentially comprises the overhead gaseous stream from zone 9 plus any material vaporized in zone 13. Zone 13 is typically operated at a relatively low temperature and pressure as compared to zone 9. In fact excellent results are obtained at a temperature of about 125 C., a pressure of about 25 p.s.i.g. and a mole ratio of entering CO to entering sulfide of about 1.111. Although satisfactory results can be obtained by stripping at relatively low temperatures under non-boiling conditions it is, of course, advantageous to use conventional means such as a stream coil or reboiler to heat the liquid in the bottom portion of zone 13 in order to further generate up-flowing vapors which aid in the liberation of H 8. Typically about 10 to 30% of the water entering zone 13 can be taken overhead with the H 5.

An overhead gaseous stream is then withdrawn from zone 13 via line 14 and passed to the junction of line 17 with line 14. The major portion of this overhead stream is then withdrawn from the system via line 17. The gas stream withdrawn via line 17 contains the net sulfide product of the present process and, it can be charged to any suitable process for the recovery of sulfur or the manufacture of sulfuric acid if desired; for example, this stream could be passed to a conventional Claus unit for recovery of sulfur via an oxidation procedure. This overhead gaseous stream contains a relatively large amount of hydrogen sulfide and carbon dioxide and minor amounts of carbon monoxide and water. Another portion of this overhead stream is passed via line 14 to zone 6 in order to supply the hydrogen sulfide reactant (i.e., the second reducing agent) necessary for the conversion of sulfite to thiosulfate in zone 6 as previously explained.

A stream of regenerated absorbent is withdrawn from the lower region of zone .13 via line 15. At the junction of line 20 with line 15, a portion of this regenerated absorbent stream can be withdrawn from the system in order to remove any water product that is not removed from the system via the treated gas stream and the H 8 product stream. In addition, line 20 can be used if desired to remove a drag stream containing the minor amount of sodium sulfate by-product if it has not already been eliminated as discussed hereinbefore. The major portion of regenerated absorbent is then passed back to zone 2 via lines 15 and 4. This regenerated absorbent stream primarily contains a mixture of sodium carbonate and bicarbonate with minor amounts of unreacted sodium thiosulfate, unreacted sodium sulfite, sodium hydrosulfide and sodium sulfate. The total sulfur content of this regenerated absorbent stream is less than 10% of the total sulfur content of the rich absorbent stream withdrawn from the scrubbing section of the system via line 5. Moreover, the amount of undesired sodium sulfate formed in the scrubbing and regeneration sections of the system is less than 10% of the S0 charged to the system via line 1. Thus the SO -scrubbing process of the present invention enables the continuous scrubbing of S0 from the gas stream containing both S0 and O entering the system via line 1 with continuous regeneration and recirculation of absorbent in a closed-loop manner. In addition, the amount of undesired sodium sulfate formed in the scrubbing and regeneration sections of the system is held to extremely low levels.

It is intended to cover by the following claims all changes and modifications of the above disclosure of the present invention that would be self-evident to a man of ordinary skill in the gas treating art.

I claim as my invention:

1. A process for treating an input gas stream containing S0 and O in order to continuously remove S0 therefrom, said process comprising the steps of:

(a) contacting the input gas stream with an aqueous absorbent containing an alkaline reagent and a first reducing agent selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof, at scrubbing and thiosulfate production conditions selected to result in a treated gas stream containing a reduced amount of S0 and in an effluent water stream containing a mixture of a water-soluble sulfite compound and a water-soluble thiosulfate compound;

(b) reacting at least a portion of the eflluent stream from step (a) with a second reducing agent, selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof, at thiosulfate produc- 17 tion conditions selected to form a substantially sulfite-free efiiuent stream containing a thiosulfate compound;

(c) reacting at least a portion of the efiiuent stream from step (b) with carbon monoxide at reduction conditions selected to produce a sulfide-containing aqueous efiiuent stream;

(d) stripping hydrogen sulfide from at least a portion of the aqueous effiuent stream from step (c) to produce a regenerated aqueous absorbent stream and an H s-containing overhead stream; and thereafter,

(e) passing at least a portion of the resulting regenerated aqueous absorbent stream to step (a).

2. A process as defined in Claim 1 wherein the alkaline reagent utilized in the absorbent is ammonium carbonate or ammonium bicarbonate or ammonium sulfite.

3. A process as defined in Claim 1 wherein the alkaline reagent utilized in the absorbent is ammonium hydroxide.

4. A process as defined in Claim 1 wherein the alkaline reagent utilized in the absorbent is an alkali metal hydroxide or carbonate or bicarbonate or sulfite.

5. A process as defined in Claim 4 wherein said alkali metal is sodium.

6. A process as defined in Claim 4 wherein said alkali metal is potassium.

7. A process as defined in Claim 1 wherein the alkaline reagent utilized in the absorbent stream is an alkaline earth metal hydroxide or carbonate or sulfite.

8. A process as defined in Claim 1 wherein the thiosulfate production conditions utilized in step (b) include a temperature of about 20 to about 150 C. and a pressure suflicient to maintain the effluent stream from step (a) in the liquid phase.

9. A process as defined in Claim 1 wherein the reduction conditions utilized in step (c) include a temperature of about 100 to about 350 C. and a pressure sufficient to maintain the effluent stream from step (b) in the liquid phase.

10. A process as defined in Claim 1 wherein the amount of carbon monoxide charged to step (c) is sufficient to provide a mole ratio of carbon monoxide to thiosulfate of at least 4: 1.

11. A process as defined in Claim 1 wherein the first reducing agent is obtained by passing a portion of the efliuent stream from step (c) to step (a).

12. A process as defined in Claim 1 wherein the first reducing agent is obtained by controlling the stripping conditions used in step (d) so that sulfide remains in the regenerated aqueous absorbent stream.

13. A process as defined in Claim 1 wherein the amount of the first reducing agent passed to step (a) is sufficient to convert 10 to 90% of the absorbed S0 to thiosulfate.

14. A process as defined in Claim 1 wherein the first reducing agent is a portion of the H S-containing overhead stream from step (d).

15. A process as defined in Claim 1 wherein the second reducing agent is hydrogen sulfide.

16. A process as defined in Claim 15 wherein at least a portion of the hydrogen sulfide utilized in step (b) is obtained by passing to step (b) a portion of the H 5- containing overhead stream formed in step (d).

17. A process as defined in Claim 1 wherein the second reducing agent is finely divided sulfur which is used in an amount at least sufiicient to provide a mole ratio of sulfur to sulfite of 1:1.

18. A process as defined in Claim 1 wherein the second reducing agent is a polysulfide compound which is used in an amount at least sufiicient to provide a mole ratio of polysulfide to sulfite of 1:6.

19. A process as defined in Claim 1 wherein the second reducing agent is a water-soluble sulfide compound which is used in an amount at least sufiicient to provide a mole ratio of sulfide to sulfite of 1:2.

20. A process as defined in Claim 1 wherein step (c) is performed in the presence of a catalyst comprising a catalytically effective amount of a metallic component selected from the group consisting of the transition metals of Groups VI and VIII of the Periodic Table combined with a porous carrier material.

21. A process as defined in Claim 20 wherein the catalyst is a catalytically effective amount of cobalt sulfide combined with activated carbon or alumina.

22. A process as defined in Claim 1 wherein step (d) is performed with a stripping gas comprising carbon dioxide.

References Cited UNITED STATES PATENTS 3,574,097 4/1971 Urban 423514 3,644,087 2/1972 Urban 423--5 14 X 3,728,433 4/ 1973 Urban 423242 GEORGE O. PETERS, Primary Examiner US. Cl. X.R. 423514, 564 

